Piezoelectric ceramic-polymer composites and piezoelectric ceramics are used in transducers for medical ultrasound imaging. In medical ultrasound there is an increasing need to improve the imaging range and resolution, as determined by the sensitivity and bandwidth of the transducer. Piezoelectric composites enjoy at least three major advantages over piezoelectric ceramics. These advantages are reduced specific acoustic impedance (Z), increased thickness coupling (k.sub.t), and reduced planar coupling (k.sub.p). As a result, piezoelectric composite transducers provide medical imaging with better axial and lateral resolution.
For high frequency applications, such as ultrasonic imaging, the piezoelectric ceramic elements in the composite must have extremely small dimensions for sufficiently high frequencies to be attained. The fineness of these composite array elements makes composite manufacturing extremely difficult.
Presently, piezoelectric composites having 1-3 connectivity are commonly used in medical ultrasound transducer applications. 1-3 connectivity composites are commonly used because of the significantly reduced planar coupling constants and reduction in the lateral coupling modes that can be achieved over those that can be achieved by using homogeneous ceramic or isotropic 3-3 connectivity composites of the same materials.
A 1-3 connectivity composite is one where one phase, typically the ceramic phase, is self connected in one direction (Z direction or thickness direction) of the composite, while the other phase, typically the infiltrate phase, is self connected in three directions (X, Y, Z directions) of the composite. Several attempts have been made to demonstrate viable manufacturing processes for 1-3 connectivity composites. A procedure has been developed for assembling composites from extruded piezoelectric fibers using automated fiber placing and assembly. This approach is effective for coarse composites having fibers of approximately 0.5-1.0 millimeter diameter, which are strong enough to be machine handled. For finer scale composites, a typical practice is to dice the ceramic composite structure from solid ceramic using a wafer dicing saw. In this case, a portion of the solid ceramic piece is left in tact as a support for the piezoelectric fiber array. This technique is referred to as dice and fill. In a 1-3 connectivity composite, the minimum dimensions of a ceramic rod or fibers and the spaces are limited by the dicing blade and the mechanical strength of the ceramic. As a result, 1-3 composites do not always satisfy the increasing need for high frequency transducer operation in medical ultrasound applications.
It is desirable to have a manufacturing method to fabricate piezoelectric composites that can be used as electromechanical devices, such as transducers, that have a structure of an interconnected lamelli and an interconnected interlamellar region which can be processed into an anisotropic 3-3 connectivity composite having improved electromechanical properties. It is also desirable to have a method of fabricating a piezoelectric composite that can be used in an electromechanical device having reduced planar coupling, a high thickness coupling constant, and reduced specific acoustic impedance.